When I want to see the differences between a branch and its parent, I find myself often preparing one directory at the branch leaf, and the other at the pivot with the parent:
<verbatim>
cd mybranch; fossil update mybranch; cd ..
cd pivot; fossil test-find-pivot mybranch parentbranch; fossil update XXXXX
meld mybranch pivot  # a program to show the differences between trees
</verbatim>

I'd like to be able to run:
<verbatim>
fossil update pivot:mybranch:parentbranch
</verbatim>